Please refer to FIG. 1, which shows the schematic circuit diagram of a conventional PWM buck DC/DC converter in the prior art. In FIG. 1, the buck DC/DC converter 100 includes a switch 101, an input capacitor 102, a main choke 103, a main diode 104, an output capacitor 105 and a load, and an input DC voltage V1 is lowered to make an output DC voltage V2 generated on the load through turning on and off of the switch 101.
The operational principles of the buck DC/DC converter 100 of FIG. 1 are that the input DC voltage V1 is employed to charge the main choke 103 and the output capacitor 105 to store the electrical energy and the main diode 104 is turned off due to a reverse bias voltage when the switch 101 is turned on firstly. The main diode 104 is turned on and the energy stored in the main choke 103 is employed to charge the output capacitor 105 when the switch 101 is turned off secondly. And the switch 101 is turned on and off periodically so as to make the output DC voltage V2 generated on the load thirdly.
The operational principles of the buck DC/DC converter 100 of FIG. 1 are that the input DC voltage V1 is employed to charge the main choke 103 and the output capacitor 105 to store the electrical energy and the main diode 104 is turned off due to a reverse bias voltage when the switch 101 is turned on firstly. The main diode 104 is turned on and the energy stored in the main choke 103 is employed to charge the output capacitor 105 when the switch 101 is turned off secondly. And the switch 101 is turned on and off periodically so as to make the output DC voltage V2 generated on the load thirdly.
Theoretically, if the capacitance of the output capacitor 105 is relatively quite large, then the switch 101 would be turned on and off quickly and periodically to accumulate the energy in the main choke 103 so as to supplement the energy on the output capacitor 105 at any time as shown in FIG. 1. Thus, the voltage across the output capacitor 105 will be maintained at a fixed value and won't be influenced by the variation of the load.
However, the buck DC/DC converter 100 has at least two drawbacks:                (1) The reverse recovery current on the main diode 104 will make the switch 101 and the main diode 104 produce serious switching losses when the switch 100 is turned on and off such that the size of the main choke 103 of the buck DC/DC converter 100 could not be lowered down through increasing the switching frequency.        (2) The voltages and currents of the switch 101 and the main diode 104 vary dramatically during the switching procedures, which will cause the EMI/RFI problems.        
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived the soft-switching DC/DC converter having relatively better effectiveness.